1. Field of the Invention
The present invention relates to a field emission display device having carbon nanotubes and a method of fabricating the same, and more particularly, to a field emission display device in which an alignment error between a gate electrode and a cathode electrode due to high-temperature firing does not occur, and a method of fabricating the same.
2. Description of the Related Art
Display apparatuses used for personal computers (PCs) and television receivers include cathode-ray tubes, liquid crystal displays (LCDs), plasma display panels (PDPs), and field emission displays (FEDs), which use high-speed thermionic emission.
FEDs using carbon nanotubes is much more advantageous than cathode-ray tubes in a wide view angle, high resolution, low power, and temperature stability. Such FEDs can be applied to various fields such as car navigation apparatuses and view finders of electronic video equipment. Particularly, FEDs can be used as alterative display apparatuses for PCs, personal data assistants (PDAs), medical instruments, high definition television (HDTV), and so on.
FIG. 1 is a diagram showing the structure of a conventional field emission display device. Referring to FIG. 1, the conventional field emission display device includes a substrate 1; an indium tin oxide (ITO) electrode layer 2 formed on the substrate 1; a mask cathode layer 3 formed on the ITO electrode layer 2 such that the ITO electrode layer 2 is partially exposed; an insulation layer 5 formed on the mask cathode layer 3 such that a well 8 is formed; a gate electrode 6 formed in the shape of a strip on the insulation layer 5; and an electron emitting source 31 including carbon nanotubes formed on the ITO electrode layer 2 exposed at the bottom of the well 8.
FIGS. 2A through 2J are diagrams showing the stages in a procedure of forming a triode structure before printing carbon nanotube paste in a conventional method of fabricating a field emission display device.
As shown in FIG. 2A, the ITO electrode layer 2 is formed on the substrate 1, and the mask electrode layer 3 is deposited on the ITO electrode layer 2. The substrate 1 is made of glass, and the mask cathode layer 3 is made of a material such as a metal or amorphous silicon which blocks ultraviolet rays.
As shown in FIG. 2B, photoresist 11-1 is deposited on the mask cathode layer 3; a mask 71-1 is disposed above the mask cathode layer 3; and ultraviolet rays are radiated for exposure. After exposure, etching and cleaning are performed, thereby forming a hole 4 in the mask electrode layer 3, as shown in FIG. 2C.
As shown in FIG. 2D, the insulation layer 5 is formed on the mask cathode layer 3 and is then fired at a temperature higher than 550° C. for an insulation characteristic. Thereafter, the gate electrode 6 is deposited on the insulation layer 5, as shown in FIG. 2E.
FIG. 2F shows a photoprocess including exposure, development, etching, and cleaning for patterning the gate electrode 6. Reference numeral 71-2 denotes a mask, and reference numeral 11-2 denotes photoresist. If the photoprocess is completed, the gate electrode 6 having a hole 7, as shown in FIG. 2G. Thereafter, wet or dry etching is performed to etch the insulation layer 5 and the mask cathode layer 3, thereby forming the well 8 such that the ITO cathode layer 2 is partially exposed at the bottom of the well 8, as shown in FIG. 2H.
As shown in FIG. 21, after photoresist 11-3 is deposited and a mask 71-3 is disposed, a photoprocess is performed, thereby patterning the gate electrode 6 in the shape of a strip, as shown in FIG. 2J.
In the above-described conventional method of fabricating a field emission display device, the substrate 1 made of glass may be deformed by the heat during high-temperature firing, so an alignment mark may be displaced. Due to displacement of the alignment mark, the center of the hole 4 of the mask cathode layer 3 does not exactly meet the center of the well 8 after the gate electrode 6 is deposited and patterned, as shown in FIG. 21. As a result, the electron emitting source 31 is displaced from the center of the well 8 to the right or left. Due to an alignment error between the gate electrode 6 and the electron emitting source 31, the gate electrode 6 may become in contact with or very close to the ITO cathode layer 2, resulting in current leak or a decrease in the amount of electrons emitted.
FIGS. 2K through 2Q are diagrams showing the stages in a procedure of making carbon nanotubes into an electron emitting source in the triode structure formed by the procedure including the stages shown in FIGS. 2A through 2J in the conventional method.
In injecting carbon nanotube paste into the well 8, a lift-off method using a sacrificial layer, a method of performing direct alignment and injecting carbon nanotube paste, or a rear exposure method can be used. When the method of performing direct alignment and injecting carbon nanotube paste is used, it is difficult to achieve a high aspect ratio due to an alignment error in equipment and viscosity of a carbon nanotube material. In the rear exposure method, since a sacrificial layer is not used, a large amount of residues are produced.
Accordingly, a lift-off process using photoresist as a sacrificial layer is generally used, as shown in FIGS. 2K through 2Q, in fabricating an electron emitting source using carbon nanotube paste.
Referring to FIG. 2K, photoresist 11-4 is deposited and a mask 71-4 is disposed on the substrate 1 having a triode structure shown in FIG. 2J such that the well 8, the insulation layer 5, and the gate electrode 6 are covered with the photoresist 11-4. Thereafter, a photoprocess is performed, thereby etching the photoresist 11-4 only formed in the well 8, except for the photoresist 11-4 formed on the insulation layer 5 and the gate electrode 6, as shown in FIG. 21.
After the etching step, as shown in FIG. 2M, carbon nanotube paste 12 is injected into the well 8 by a screen printing method and is deposited on the entire surface of the photoresist 114, and then rear exposure is performed. Here, the photoresist 114 is used as a sacrificial layer.
If the rear exposure is completed, as shown in FIG. 2N, the carbon nanotube paste 12 is divided into exposed carbon nanotube paste 13 and non-exposed carbon nanotube paste 13′. This happens because the carbon nanotube paste 13′ positioned in front the mask cathode layer 3 is not exposed to ultraviolet rays.
Thereafter, development using a developer such as acetone or Na2CO3 (0.4% wt) is performed. As a result, the exposed carbon nanotube paste 13 remains, but the non-exposed carbon nanotube paste 13′ is lifted off simultaneously with diffusion of the photoresist 11-4 as a sacrificial layer to the developer, so carbon nanotube paste 14 having a shape shown in FIG. 20 can be obtained. Here, residue 14′ of the non-exposed carbon nanotube paste 13′ may not dissolves in the developer, or some of the exposed carbon nanotube paste 13 may be exposed to the developer, so carbon nanotube paste may adhere to the gate electrode 6 or the insulation layer 5.
Thereafter, the resultant structure shown in FIG. 20 is fired at a nitrogen atmosphere at a high temperature of about 460° C., thereby shrinking the carbon nanotube paste 14 to form a shrunken carbon nanotube paste 15, as shown in FIG. 2P. Then, the surface of the carbon nanotube paste 15 is mechanically processed to reveal carbon nanotubes sunken into the carbon nanotube paste 15, thereby forming the electron emitting source 31, as shown in FIG. 2Q. The residue 14′ still remains.
The residue 14′ may adhere to the surface of the triode structure, as shown in FIG. 20, causing a defect such as a short circuit between electrodes or a diode emission due to positive voltage.
FIG. 3 shows an alignment error between the gate electrode 6 and the electron emitting source 31 in a field emission display device fabricated according to a conventional method shown in FIG. 2D. In FIG. 3, the electron emitting source 31 is displaced from the center of the gate electrode 6 to the right.